As a conventional superconducting cable, a cable has been known that has a structure in which a cable core having a superconducting conductor is housed in a heat-insulated pipe. The examples of the foregoing superconducting cable include (a) a single-core cable that houses one cable core in a heat-insulated pipe and (b) a three-core-twisted-type cable that is formed by twisting three cable cores together and by housing the twisted body in the pipe. FIG. 5 is a cross-sectional view of a three-core-twisted-type superconducting cable for the three-phase AC use. A superconducting cable 100 has a structure in which three cable cores 102 are twisted together and housed in a heat-insulated pipe 101. The heat-insulated pipe 101 has a dual-pipe structure composed of an outer pipe 101a and an inner pipe 101b, between which a heat-insulating material (not shown) is placed. The space between the pipes 101a and 101b is evacuated in vacuum. Each of the cable cores 102 comprises, from the center in the following order, a former 200, a superconducting conductor 201, an electrically insulating layer 202, a superconducting shielding layer 203, and a protecting layer 204. A space 103 enclosed by the inner pipe 101b and the cable cores 102 forms a coolant channel. An anticorrosion covering 104 is provided on the outer circumference of the heat-insulated pipe 101.
The superconducting conductor 201 and the superconducting shielding layer 203 of the cable core 102 are cooled by a coolant circulating in the space 103, so that the superconducting state is maintained. As such a coolant, liquid nitrogen is commonly known. Patent literature 1 has described a superconducting cable that (a) uses a hollow former, (b) uses liquid air as the coolant to be circulated in the former, and (c) uses liquid nitrogen as the coolant to be circulated in the heat-insulated pipe.
Patent literature 1: the published Japanese patent application Tokukai 2001-202837.